1. Field of the Invention
The present invention relates to an operational transconductance amplifier (OTA) and in particular to quickly turning off an MOS device driven by the OTA.
2. Discussion of the Related Art
An operational transconductance amplifier (OTA) receives an input voltage that controls an output current. Thus, an OTA is in effect a voltage-controlled current source. FIG. 1 illustrates a conventional circuit 100 in which an OTA 101 drives the gate of a large MOS device (in this case a PMOS transistor) 102. Note that MOS device 102 can be implemented as a composite device with multiple transistors connected in a similar manner. MOS device 102 is configured with its source connected to voltage source VDD and its drain providing an output signal Vout. Thus, in this configuration, MOS device 102 can operate as a pass device for a positive regulator.
However, the gate of MOS device 102 is difficult to drive fast. Specifically, the gate of MOS device 102 has a large parasitic capacitance, i.e. a first parasitic capacitance between the gate and the source, a second parasitic capacitance between the gate and the drain, and a third parasitic capacitance between the gate and the channel. Because some finite impedance is driving the gate of MOS device 102 (e.g. the driver and polysilicon gate resistance), the large parasitic capacitance and the finite impedance yield a time constant.
Note that OTA 101 is typically used in circuit 100 rather than another type of amplifier because OTA 101 can easily drive rail to rail. Unfortunately, OTA 101 has high output impedance and limited current drive, thereby resulting in an undesirably large time constant. In practical terms, a large time constant means that MOS device 102 is slow to turn on/off.
To improve this drive capability, an output terminal of OTA 101 is connected to the negative input terminal of OTA 101, thereby providing negative feedback. OTA 101 receives an input voltage Vin on its positive input terminal and therefore is set up as a unity gain buffer. Thus, from a small signal perspective, the transductance of OTA 101, i.e. the gm, could be used to effectively counter the parasitic capacitance by lowering impedance at the gate of MOS device 102.
Specifically, reducing the impedance at the gate of MOS device 102 increases the small signal frequency response but does not results in faster turn on and turn off time.
One way to provide a fast turn-on and turn-off time at the gate of MOS device 102 is to provide a high current for OTA 101. Notably, the gm of OTA 101 is controlled by an external current, i.e. OTA's BIAS current. As a result of the OTA's high bias current, the external current of OTA 101 is high and thus can successfully drive the gate at high speed. Unfortunately, a high bias current is not desirable for low power applications.
Therefore, a need arises for a way to quickly drive the gate of an MOS device while not consuming too much current.